1. Field
The disclosure pertains to forming an inert blanket of a cryogenic fluid at the surface of a molten metal bath within a furnace.
2. Related Art
In foundry melting operations, metals (ferrous or non-ferrous) are typically melted in electric induction furnaces. It is often advantageous to melt the metals under cover of inert gas (such as argon, nitrogen or carbon dioxide) so as to minimize or prevent exposure of the molten metal to oxygen and resultant oxidation of the metal to form metallic oxides that are deleterious to cast metal products formed from the molten metal. The inert gas cover also reduces the tendency of the molten metal to absorb gases (e.g., oxygen and hydrogen) from the atmosphere, which in turn reduces gas-related casting defects such as porosity. Other benefits of melt surface inerting include reduced slag formation, improved metal fluidity, increased furnace refractory life, and reduced need for de-oxidizers.
Electric induction furnaces are generally open-top, batch melting units. The inert gas is typically applied from this open top throughout the entire melting process. A number of different furnace inerting techniques are utilized. The two main techniques involve blowing the inert gas into the top of the furnace and dripping or pouring inert liquid (in cryogenic form) into the furnace at the open top. In certain liquid inerting techniques, an inert layer of liquid argon or liquid nitrogen is formed over the entire molten metal surface to blanket the metal from oxygen and other gases.
Liquid inerting is often desirable over gas inerting, since the liquid has a higher density than the gas and therefore is more likely to stay at or near the molten metal surface rather than being forced upward due to thermal updrafts within the furnace. However, utilizing a cryogenic liquid to provide the entire blanketing or inert layer over the molten metal surface requires a significant amount of the cryogenic liquid to maintain the blanket throughout the process, and this can result in significant and excessive increase in operating costs.
In addition, the use of liquid inerting by direct application of a cryogenic liquid directly to the molten metal surface can lead to spitting or blow out of molten metal from the furnace upon contact of the cold liquid to the hot metal surface. This can be particularly dangerous to the furnace operator, particularly in furnaces in which a high meniscus profile has developed for the molten metal with in the furnace.
It would be desirable to provide a system that is capable of achieving an effective inert layer over a molten metal surface in a furnace while minimizing the amount of inert substance that is required and thus reducing operating costs associated with the melting process.